1. Field of the Invention
The invention relates to a multipole key switch with an actuating ram longitudinally slidable against the force of a restoring spring, which actuating ram engages at least two contact bridges disposed next to each other and placed under a spring restoring force for the shorting out or, respectively, lifting off from fixed position contact fingers disposed in switching chambers, where the pretensioning force of the spring of the contact bridges generates the required contact pressure and where each switching chamber is provided with its own restoring spring.
2. Brief Description of the Background of the Invention Including Prior Art
Key switches are employed in various ways for indicating a state such as for example the final position of a machine table, the closure of a door, of a cover or of similar elements. A portion of the path of the motion of the part to be surveyed is taken directly for actuating of a switch ram. Such a single pole switch is shown in German Patent DE-PS No. 952,915. In application situations where more than one electrical circuit are to be switched, it is also known from German Patent DE-PS No. 971,322 to dispose several switches in spacial sequence following each other. In many cases this results in impermissibly large construction heights in particular in cases where the switch is employed not only for a small voltage, but also for the low voltage grid and where the required distances and creeping paths are to be maintained between the various parts relative to each part and also relative to ground.
The French Patent FR-PS No. 1,169,029 (Etabl. Labinal) also shows a key switch with switching chambers disposed next to each other, the contact bridges of which are actuable with a single actuating ram. Also each of the switching chambers is provided with its proper reset switch, which however acts only on its own switching chamber. In case a reset spring is broken in a certain switching chamber in this provision, then the switching chamber is not automatically actuated by the rest spring of another switching chamber. Therefore, such switches do not meet the safety requirements forming the base for the subject matter of the application.
The U.S. Pat. No. 3,142,732 (Texas Instruments) refers to a three-pole excess current release with three switching chambers disposed next to each other, which are connected to each other with a crossbar and which can be switched on again after release with a joint reset button. The object of such thermal switches is to cause a response of the two other switches upon a response by one switch. However, this is a completely different field of application as compared to the subject matter of the present invention and no teaching is provided referring to the assurance of the switch-off safety upon breakage of one of the reset springs of one of the key switches.
The German Patent DE-PS No. 667,361 (AEG) shows only the disposition of several contact bridges disposed next to each other at a joint crossbar in a bistable toggle switch. Again, there is no suggestion here relating to the necessary safety of switching off of a key switch.
The following documents are also known to us as relating to the further state of the art: German Patent DE-PS No. 644,081 (AEG), French Patent FR-PS No. 64,500 (SA d'Appareillage Blinde), German Patent Laid Out DE-AS No. 2,618,572 (R. Schadow), and German Patent Application Laid Open DE-OS No. 2,702,225 (Westinghouse).
Additional safety requirements exist if the key switch is employed as a safety element, for example for providing barriers at punching machines against unauthorized manual access during a machine work operation or as a door contact, where it is to be assured that in case of an opened door no further dangerous contact voltages exist. The latter requirements require more than a reset element at the switch actuating the switch, which would result in additional construction depth in case of conventional constructions.